Traditional detecting methods for plastic optical lenses are complicated to operate and very time-consuming. Therefore, a new detection method based on the magnetic levitation theory was proposed for a small plastic lens and a corresponding detection device was designed in this paper. With the detection method, the diamagnetic sample (plastic lens) in the magnetic field of paramagnetic solution was levitated by magnetizing the solution. By observing the height, the centre position and the deflection of the lens in the device, the appearance and interior homogeneity like shrinkage and internal porosity of the lens could be detected. Several experiments on the qualified lenses, the lenses with an incomplete appearance or with interior homogeneity and some of disqualified lenses were performed for verifying the feasibility of the proposed method. The experimental results show that the proposed method is precise sensitive, easy to operate and has wider application fields. The detecting accuracy for density reaches to 0.003 g/cm3 under a millimeter scale. Moreover, the method detects the interior porosity of the lens with a diameter less than 10 mm and the sensitivity to the volume of the porosity reaches to 0.017 mm3. The proposed method has great potential for plastic optical lens detection.